The present invention relates to a stirrer for glass melts, and more particularly to a stirrer comprising a stirring element and a drive shaft that are connected to one another mechanically, wherein the stirring element comprises a refractory metal or alloy, and the drive shaft comprises a non-refractory metal or alloy.
Stirring devices used for glass melts can face severe challenges. Increasing demand for glass products has focused attention on the cost and reliability of glass production processes and equipment. In industrial manufacture, glass melts are processed in melting ovens. The glass material is maintained in a melt phase by convective heat transfer, which is promoted by means of stirring devices. The mechanical reliability of the stirring device can act as a limiting factor on the time utilization of the processing equipment.
Higher production rates, resulting in lower costs, demand higher temperatures of the glass melt, which results in higher thermal stresses on the stirring devices. In addition, many specialized glass products require additives that can be particularly corrosive to the stirrer device. Finally, production specifications for glass products will not tolerate the transfer of any foreign material from the stirrer, as by erosion or the consequences of corrosion.
As a rule, the glass is melted in closed oven zones, wherein a hot corrosive gas atmosphere is formed above the melt. The stirrer for the glass melt, which is driven from outside the oven zone, passes through the lid of the oven zone and, together with the drive shaft, is primarily exposed to the oven""s atmosphere, while the actual stirring element at the end of the drive shaft is exposed to the glass melt itself.
Thus the materials which are used for the glass stirrers have to withstand the corrosive environments of the oven""s atmosphere and the glass melt. These environments are distinctly different in their corrosive nature from each other. Of course, the materials that are used are not permitted to contaminate the glass melt.
Platinum is a material that withstands all these different corrosive stresses and in no way contaminates the glass melt. The disadvantages of platinum reside in its extremely high cost. A stirrer whose surface consists entirely of platinum can therefore be used only to a limited extent in the manufacture of especially high grade glass, such as glass for the manufacture of television panels or, in the case of glass ceramic melts, for the manufacture of glass ceramic hobs and, even here, only in the form of hollow stirrer constructions or platinum-encased molybdenum cores.
Glass stirrers comprising ceramic materials readily withstand corrosion by the oven""s atmosphere, but they are less resistant to the glass melt, and they can contaminate the glass melt with ceramic particles. In addition, ceramic materials are limited in the stirrer shapes into which they can be formed, and their resistance to thermal shock is extremely poor. Thus, ceramics are of limited value as materials for glass melt stirrers.
Refractory metals and their alloys are also in use for glass stirrers. Examples of common refractory metals include molybdenum, tantalum, tungsten, chromium, columbium and rhenium. Although the corrosion resistance of molybdenum with respect to the glass melt is good and the contamination of the glass melt is negligible for the majority of applications, molybdenum is extremely reactive to air/oxygen mixtures and aggressive gaseous media. In the case of glass stirrers comprising molybdenum, the drive shaft, which is located outside of the glass melt and in the corrosive atmosphere of the oven, has to be protected against corrosion by additional procedures.
One possibility for achieving this is described in German Patent No. DE 25 00 793 A, according to which the drive shaft of a glass stirrer which consists entirely of molybdenum is surrounded by a tube which comprises platinum or a platinum alloy and which is immersed in the melt, with a small separation between the drive shaft and the tube, wherein an inert gas flows through the intervening zone between the platinum tube and the molybdenum shaft.
A disadvantageous feature in this regard is that the total length of the drive shaft, which is exposed to the corrosive atmosphere of the oven, has to be encased by the platinum tube, and this makes such a glass stirrer very expensive. In addition, flushing with an inert gas also makes the glass manufacturing process more expensive, and leads to undesired bubble inclusions in the glass melt.
Another possibility for creating a glass stirrer, which exhibits good resistance to corrosion from the corrosive atmosphere of the oven and also to corrosion by the glass melt, comprises subdividing the glass stirrer, and constructing the parts from different materials as described in, e.g., U.S. Pat. No. 3,539,691. According to this document, the actual stirring element at the end of the stirrer shaft is preferably manufactured from molybdenum, and the drive shaft as far as the stirring element and including the section that is immersed in the glass melt, is manufactured from a different material, such as copper or an iron alloy.
A disadvantageous feature in the case of such a glass stirrer is that the interface between the two stirrer parts is located within the glass melt. The section of the drive shaft that is located within the glass melt is thus subject to aggressive corrosive attack and since this section of the drive shaft comprises the material that is primarily resistant to the corrosive atmosphere of the oven rather than corrosion from the glass melt, premature failure of the glass stirrer can result. In addition, the sections of the drive shaft which are immersed in the glass melt have to be cooled internally in order to continue ensuring adequate mechanical strength for the drive shaft even at the temperature of the glass melt.
It is an object of this invention to provide a stirrer apparatus for stirring a glass melt comprising a stirring element comprising an impeller and a shaft element, both made of a refractory metal or alloy as a first metal, a drive shaft constructed of a second non-refractory metal or alloy, said shaft element and said drive shaft each having an end portion, the end portion of the shaft element being adapted to mechanically attach to the end portion of the drive shaft, and a casing made of platinum and disposed over the end portion of the shaft element or the drive shaft over a length suitable to isolate said end portions against corrosive attack within a range of surface levels of the glass melt.